What is WAV?
WAV (or WAVE), short for Waveform audio format, is a Microsoft and IBM audio file
format standard for storing an audio bitstream on PCs. It is a variant of the
RIFF bitstream format method for storing data in "chunks", and thus
also close to the IFF and the AIFF format used on Amiga and Macintosh computers,
respectively. It is the main format used on Windows systems for raw and typically
uncompressed audio. The default bitstream encoding is the Microsoft Pulse Code
Modulation (PCM) format.

What is MP4?
MPEG-4 Part 14, formally ISO/IEC 14496-14:2003, is a multimedia container format
standard specified as a part of MPEG-4. It is most commonly used to store digital
audio and digital video streams, especially those defined by MPEG, but can also
be used to store other data such as subtitles and still images. Like most modern
container formats, MPEG-4 Part 14 allows streaming over the Internet. The official
filename extension for MPEG-4 Part 14 files is .mp4, thus the container format
is often referred to simply as MP4.